Choppy HD video playback in full screen with Quicktime

I've got a HP Pavilion dv6000 notebook running Windows Vista Home Premium. It's got an AMD Athlon 64 X2 Dual Core 1.7GHz Processor TK-53 with 2GB DDR2 667MHz RAM. The graphics processor is nVIDIA GeForce Go 6150. I get very choppy HD video playback in full screen mode with Quicktime, audio works fine. Like if I try to watch a movie trailer in HD & view it in full screen the video playback is quite far from smooth-I generally use the Apple movie trailer website which loads Quicktime separately for HD videos. However, if I 'maximize' the window but don't view it in full screen the video plays perfectly smooth. Or, if I save the video to my computer & then view it with VLC Media Player or Windows Media Player, it works perfectly fine. This is with all the resolutions; 480p, 720p & 1080p. 1080p doesn't work well with Windows Media Player though, only VLC Media Player is able to play that smoothly. Why is this happening, how do I fix it? I've already tried the 'safe mode (GDI only)' option which didn't change anything.
Is there anyone with the same/similar configuration, mainly the same GPU that can play those HD videos on full screen through Quicktime without any problems? If so, I would like to know which driver you've got installed. I know it's not the hardware being incapable cause a friend of mine with the Apple Macbook (with the GMA 950) managed to play 1080p trailers in full screen without any problems.